Uterine chromatin template activity during the early stages of pregnancy in the rat.

1. The template activity of chromatin prepared from rat uterine nuclei during dioestrus, oestrus and the first 7 days of pregnancy has been examined. 2. The DNA, RNA, histone and non-histone protein contents of uterine chromatin remained constant during early pregnancy. 3. The rate of RNA synthesis on Day 1 uterine chromatin was 8.61 +/- 0.59 (mean +/- S.E.) pmol of UMP incorporated/mg DNA per 10 min. When compared with DNA prepared from rat liver nuclei, 13.20 +/- 0.27% (mean +/- S.E.) of the Day 1 chromatin DNA was available for transcription by Escherichia coli RNA polymerase. 4. Uterine chromatin from rats in early dioestrus had significantly less template activity than during oestrus. 5. Chromatin prepared from whole uterus on Day 5 and from implantation sites on Days 6 and 7 of pregnancy had a significantly higher template activity than chromatin obtained from uteri on Day 1. Chromatin from interimplantation tissue on Day 6 had a lower template activity than that from uteri on Day 1. 6. RNA - DNA hybridisation of RNA transcribed from chromatin obtained on Days 2, 5 and 7 of pregnancy showed that RNA transcribed from Day 5 chromatin obtained species not present (or present in very small amounts) in RNA transcribed by chromatin from uteri on Day 2 and from implantation tissue on Day 7 of pregnancy. 7. The results are discussed in relation to the cellular changes occurring in the stroma immediately before implantation and it is postulated that the appearance of a new species of RNA on Day 5 is related to the preparation of the stromal cells for decidualisation.